Field of the Invention
The present invention relates to a gearbox of a power unit that can be downsized, and includes a transmission, a shift drum provided with lead grooves, and multiple shift forks engaging with the lead grooves.
Description of Related Art
Japanese Patent No. 4906596 discloses a structure of a gearbox of a power unit aimed to downsize an internal combustion engine (power unit), which includes a gearbox having a transmission and other parts. The gearbox includes a constant mesh gear transmission as a shift mechanism and a shift drive mechanism, which includes a shift drum and shift forks, configured to drive the shift mechanism. In the shift drive mechanism, shift pins of two shift forks, which sandwich the shift drum in the longitudinal direction, are both engaged with one of lead grooves formed on the outer surface of the shift drum. For this reason, there is no need to provide a lead groove for each shift fork. Hence, the number of lead grooves in the shift drum can be reduced, whereby the axial length of the shift drum is shortened, and the entire power unit is downsized.
The lead groove of the shift drum is provided with a circumferential groove portion extending along the circumferential direction of the shift drum, and axially shifted groove portions that are shifted in the axial direction of the shift drum from the circumferential groove portion. When the shift drum is rotated, the shift forks are guided by the lead groove to move in the axial direction of the shift drum, and establish speeds of the transmission.
In the structure disclosed in Japanese Patent No. 4906596, the two shift pins engaging with the single lead groove are guided by the same circumferential groove portion. Meanwhile, the axially shifted groove portions for guiding the two shift pins are provided in positions shifted in different directions, and the two shift pins do not share a single axially shifted groove portion in the structure. Hence, axially shifted groove portions for moving the respective shift forks are provided separately in the lead groove, which inhibits downsizing of the shift drum in the radial direction. Accordingly, there still remains room for improvement in downsizing of a gearbox including a shift drum, and downsizing of an internal combustion engine (power unit) including the gearbox.